1. Field of the Invention
The present invention relates to a clamping force sensor system. More particularly, the present invention relates to a clamping force sensor system that is capable of detecting and displaying clamping forces.
2. Description of the Related Art
In mold-clamping operation of a typical injection molding machine or die casting machine, plastic material is fed into a mold chamber of a mold, and a movable clamping cap is moved to carry out opening/closing of the mold. The pressure applied to the mold decides formation precision of the plastic material whereas the product quality largely depends on the evenness of the clamping forces. Hence, detection of the clamping forces is required and adjustments based on the measurement results may proceed.
Four tie bars are used in a typical injection molding machine or die casting machine of either vertical type or horizontal type. FIGS. 1 and 1A of the drawings illustrate a conventional meter 11 mounted near a rear end of each tie bar 12 of a forming machine 2. When the tie bar 12 has any minor change in its overall length during the process of clamping the mold to touch a probe 13 of the meter 11, the probe 13 would contact with and press against a spring (not shown) inside the meter 11, so that the amount of axial stretch of the tie bar 12 may be read from a pointer 14 of the meter 11. The axial force (i.e., the mold-clamping force) applied to the tie bar 12 can be calculated by one or more equations. However, the conventional meter 11 has low accuracy, and the spring inside the meter is subject to elastic fatigue after being used over a long time. Further, an operator has to visually observe the reading of the axial stretch amount of the tie bar 12 on the conventional meter 11. Errors might be caused due to different observation angles and personal subjective judgment or negligence, leading to inaccurate data.
FIGS. 2 and 2A show another conventional strain meter 21 for measuring the clamping force. The strain meter 21 mainly includes a sensing element 211 having two signal lines 212 electrically connected to a monitor 23, so that a signal from the sensing element 211 may be processed by an operation unit and amplified by an amplifier unit, and finally displayed on the monitor 23. This type of strain meter 21 is flatly adhered to an outer surface of each tie bar 22 of a forming machine 2. When a movable clamping cap slides on the tie bars 22 relative to a fixed clamping cap to open or close the mold, the sensing element 211 is able to detect a change in the electric resistance of the tie bars 22 due to the axial stretch thereof, thereby measuring the clamping force at the time the mold is closed. However, the costly high-precision strain meter 21 must be discarded, for the sensing element 211 is damaged when it is removed from the outer surface of the tie bar 22. Further, it is troublesome, time-consuming, and difficult to adhere the sensing elements to the tie bars 22 at uniform angle and tightness, and therefore have adverse influence on the measurements. Sandpapers and pre-installation cleaning jobs are required.
U.S. Pat. No. 6,935,188 discloses a surface-mounted type clamping force strain meter associated with a locating seat. U.S. Pat. No. 6,941,820 discloses a cuff type clamping force sensor. In these patents, a clamping force sensor includes a locating seat mounted adjacent to a tie bar, with a portion of a sensing element in contact with an outer surface of the tie bar for detecting a change in the electric resistance of the tie bar, thereby detecting the stretch. However, the signal is an electric signal regarding the resistance. Although the clamping force can be obtained by converting the electric signal into the corresponding value in terms of stretch and then checking the Yong's modulus and a load/strain chart, the procedures are troublesome and time-consuming.